Basics for Wiring a 3 Phase Motor Terminal Box

Updated February 21, 2017

Three-phase motors are used almost exclusively on three-phase power. There are two voltages in which the three-phase motors operate: 240 VAC and 480 VAC. In almost all applications, three-phase motors are much more efficient than single-phase motors of the same horsepower rating. Basic factors must apply when wiring these motors to the feed or supply voltage.

Wire Size and Protection

Protect all wires that are feeding voltage to the motor in an approved conduit or enclosure. This can include metal pipe and flexible motor conduit. Failure to adequately protect the wire from mechanical damage can cause serious injury to personnel. Size the wire to the motor's nameplate data tag. All motors will have a metal tag attached to the side or top of the motor frame. This will include the operational voltage and amperage. Make the wire size slightly larger than the rating, if the amperages are close to the maximum size. In other words, if a motor is rated for 20 amperes, you may want to use 10-gauge wires, which are rated for carrying 30 amperes. Consult all local electrical regulations for exceptions and rules to any motor wiring project.

Terminations and Connections

Inspect the motor terminal box for any signs of damage or missing seals. All terminal boxes must have a seal to keep all forms of moisture from entering the wiring area. Boxes that have corrosion or small holes must be replaced before any connections can occur. Failure to do so can allow moisture or foreign debris to enter the motor and create premature failure to the device. Use only approved-sized insulating connectors and wire nuts. Covering large wires with an undersized insulating connector will cause that connector to overheat and fail. Melting of the insulator will cause the electrical power to short against any metal portion of the motor. This will cause the electrical circuit to trip breakers and blow protective fuses. In some cases, it can even damage the motor itself.


Three-phase motors will have various motor wiring patterns depending on the manufacturer. Even motors of the same voltage, horsepower and application can have different wires leading to the motors windings. Check the nameplate data tag for any versions of that particular motor for wiring. A motor can have six to nine leads that conduct the electrical power, placed inside the terminal wiring box, to the motor's copper windings. Some motors will have up to 12 wire leads for special applications. Check with all specifications if you are unsure about these differences.

Motors that contain internal thermostats for tripping the motor control circuit will have to be wired accordingly. Other electric motors may also use an internal braking system built into the three-phase motor. Failure to correctly terminate internal brakes on motors will cause the motor to prematurely fail and thus be damaged before it even has a chance to operate.

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